LAS VEGAS (AP) — Health officials urged tuberculosis testing for hundreds of babies, family members and staff who were at a Las Vegas neonatal intensive care unit this past summer, saying they want to take extra precautions after the death of a mother and her twin babies and the infection of more than 26 people.

Authorities with the Southern Nevada Health District said Tuesday that they’re working to contact parents of about 140 babies who were at Summerlin Hospital Medical Center’s NICU unit between mid-May and mid-August, and have set up a temporary clinic to test them.

Tests of hospital staff and friends and family of the mother have revealed 26 TB infections, although most of those cases are latent — meaning patients don’t show symptoms and aren’t contagious. All 26 are being treated, according to health district spokeswoman Stephanie Bethel.

It’s unlikely that the babies who may have been exposed will come down with the disease, according to Dr. Joe Iser, the health district’s chief medical officer, but officials want to do widespread testing “through an abundance of caution.”

The 25-year-old mother then gave birth to extremely premature babies in the Las Vegas area in early May. One baby, 3-week-old Emma White, died June 1 of respiratory failure and extreme prematurity, according to the Clark County coroner.

She was never tested for TB.

The mother, who had been sick before and after the birth, was eventually admitted to a Las Vegas hospital, and later transferred to a Southern California hospital “for a higher level of care,” according to an Aug. 22 report from the health district. The mother died in California and her name was unavailable. An autopsy showed she had tuberculosis meningitis.

The second baby, Abigail White, was tested for TB and treated, but she succumbed to the disease at Summerlin Hospital on Aug. 1.

The delayed diagnosis may have come because the disease is relatively rare in the U.S.

“TB can be very subtle, particularly as many doctors have not had much experience with TB these days,” said Dr. William Schaffner, professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine.

Families who had babies in the NICU were first informed of the investigation in August, but it wasn’t until this week that the health district has urged them to get tested. Before that, health officials tested more than 200 hospital staff members and 69 close family and friends.

“We did a full contact investigation and that helped us to determine her movements during the time she was not hospitalized but likely infectious, and we’ve followed up with all the contacts we know,” Iser said.

Officials from the Centers for Disease Control and Prevention assisted with the probe up until last week, Bethel said. It wasn’t immediately clear whether their investigation has been affected by the government shutdown.

TB can be fatal if not properly treated. The CDC counted 569 TB deaths in the U.S. in 2010, the most recent year for which data is available.

The illness is spread through the air when a sick person coughs, sneezes or speaks. Symptoms include coughing, chest pain, fever and fatigue.

Schaffner said dairy cattle can sometimes carry the disease and then pass it along through unpasteurized milk, but he contends that’s extremely rare, especially in the U.S. Virtually all cases are transferred from person to person, he said, and TB occurs far more often in people who were born overseas.

The number of U.S. TB cases has been on a steady decline since a resurgence in 1992, and in 2012 reached the lowest level since national reporting began in 1953.